Japanese Patent No. 4515601 discloses a cardboard-made tray provided with closure flaps which cover a portion of an upper end opening of a tray body. In such a tray, fruits or vegetables are stored in the tray body in a state where the closure flaps are outwardly inclined. After the fruits or vegetables are stored, the closure flaps are folded in a horizontal direction, and inserting projections of locking pieces formed on both side edges of the closure flaps are inserted into insertion holes formed in the tray body, thus assembling the tray.
However, in assembling the tray disclosed in Japanese Patent No. 4515601, it is necessary to hold the closure flaps and the locking pieces, and to position and insert the inserting members of the locking pieces into the insertion holes with an operator's fingers while maintaining such a state, complexing an assembling operation. Further, it is extremely difficult to automatically assemble such a tray which requires a complex assembling operation using an apparatus, and the apparatus requires a complicated mechanism so that a cost is pushed up.